How to make grooves without editing geometry
You can add a groove using a boolean modifier.

My process would be as follows:

Duplicate some faces from your original, then separate them into another object.



Scale them down, and extrude out some thickness to get the inverse of the groove you want:



Add a boolean modifier on the first object set to difference, and select the new object as the target:



Then, just hide the second object to see the groove: